Sevier & Laramide Orogenies

In several of my web site pages I have discussed plate tectonics, in particular I have mentioned that older rocks being thrust over younger rocks in Nevada, especially in Red Rock, were the result of the Pacific plate colliding with the continental plate, moving down underneath it (subducting) and in the process putting sufficient pressure on the terrain to cause huge chunks of it to break and ride up and over the existing (younger) rocks.  That explained the Keystone thrust fault in the Red Rock Canyon National Conservation Area just west of Las Vegas, for example.

Turns out that is a great story, a story currently still told on the Bureau of Land Management's web site describing Red Rock, in fact, where the evident thrusting is explained in this fashion:

So, if I keep quiet I am in good company telling the story the way I have been telling it except for one thing: I said the limestones were 300 million years old and the underlying sandstones 200 million years old.  The age of the sandstones is OK, but according to the Bureau of Land Management explanation the limestones are around 600 million years old and did not come from the Pacific Ocean floor south of the equator, as I said, but were locally deposited because about that time the west coast was where Utah is now.  They are local, and Cambrian in age, in other words.  

BUT: there is a minor problem with this thrusting being caused by "compressional forces driving one crustal plate over another."  Obviously there were large chunks of terrain pushed eastward with exactly that effect, but these should not be called plates in the geologic sense of the word.  The problem is that there was no plate riding on top of the existing terrain, there was another driving force at work that resulted in building mountains. Mountain building, or causing mountains to form, is the meaning of the word orogeny.

There are mountains, like the Himalayas, that are the direct result of the collision of two continental plates,  but that is not what was happening here in Nevada.  The orogenies that, in stages, built the mountains from the Colorado through Utah and eastern California were not the direct result of plate collisions.  The one of greatest interest "locally" are the Sevier and the Laramide orogenies, which overlapped in time and areas affected.  They may have been the result of a plate moving rather shallowly far under the continental plate before it took a downward turn.  A plate in this sense is a huge area of rock that came to the surface in an almost molten state in the spreading zone at the center of the Pacific and as more material comes out of the spreading zone the plate is continually pushed away in both directions (eastward and westward).  In this instance, a plate called the Farallon plate stayed shallow and dove down rather farther east than others had done before it finally sank down.  This caused stresses and moved surface terrain because it became attached to (cemented to, as it were) some of the deeper roots of that surface terrain as it moved, moving terrain on top of the existing terrain and making thrust faults like the Keystone fault.

If that is the real explanation for the Keystone thrust fault in Red Rock, isn't the Bureau of Land Management explanation "close enough" since it makes a general statement about plate collisions being driving forces?  

But here is where things get exciting:  the Sevier and larger and later Laramide orogenies are considered to be less than satisfactorily explained. The explanation I just walked you through is the generally accepted one, accepted by people like me who do not make their living thinking about these things.  

But geologists who actually do make their living thinking about these things have called these orogenies "enigmatic," they are largely unexplained.  

Isn't there some proof for the Farallon shallow trajectory before subduction idea?  No.  That plate was subducted and reabsorbed into the mantle long, long ago.  So, there is no way to directly test the Farallon plate hypothesis, and indirectly there are some observations that do not fall neatly into that hypothesis.

So, obviously, because geologists are born as curious as anyone, if not more so, there are other hypothesized explanations: quite a few in fact but I will only describe three.   Two are very different from the Farallon plate theory/hypothesis, and one is an interesting variant on the Farallon plate hypothesis.  These three I liked largely because I think I understood them.

ALTERNATIVE HYPOTHESIS 1

This is my personal favorite: the “Hit-and-run collision model” by Julie A. Maxson, and Basil Tikoff, of Carleton College and the University of Minnesota. (Published in Geology (Boulder) November 1996, 24(11):968-972.) They postulate that the formation of the Rocky Mountains may have been caused by a collision of a land mass near Baja California that caused the compression of the west.  The colliding land mass “hit” and then ‘ran’by turning northwest where it came to a halt as the western part of British Columbia and southeast Alaska.  This occurred somewhere between about 94 and 40 million years ago, and the time of the Laramide orogeny was from about 80 to 45 million years ago, a highly suggestive coincidence.  

I like this theory or conceptual model simply because I can picture it in my mind.  It must have exerted compressive pressure all the way up the coast from Baja, of course, since mountain building during that time ranged from Baja to Alaska, although the coastal ranges are not at issue here, they are the product of the ever-continuing subduction of Pacific plates, one after the other, under the continental mass they encounter, with the formation of the “Ring of Fire” all around the Pacific.

ALTERNATIVE HYPOTHESIS 2

Yet another theory has no plates colliding but instead has a previously buckled and compressed plate relaxing and becoming flatter, and as its central mass moves down it sucks the overlying terrain down and pulls from each side, causing a compression locally that forms mountains made of the lighter rocks that would not easily be pulled down.  That is my simplification of what was presented in a scientifically astute manner by L. J. O’Driscoll and two co-authors from the University of Oregon.  Their presentation was entitled “Cause for Andean and Laramide type orogeny” and was abstract #T33F4-04 in the American Geophysical Union Fall Meeting in 2006.   They model a flattening plate and all the stresses that would be part of such an event.  Their results have use in explaining what is seen in the Andes and the Rockies without resorting to any plate collisions, whether the collision results in overriding existing land or pushing it up by riding underneath it.   It has an obvious appeal as a hypothesis, not only because of its simplicity but also because two large mountain ranges can be explained by this hypothetical cause.  It would be a real challenge to test aspects of this hypothesis, I would think, but that is probably true for the main hypothesis and all three of the alternatives discussed here.

ALTERNATIVE HYPOTHESIS 3

Another rather appealing alternate hypothesis was presented at the 2007 Geological Society of America Annual Meeting in Denver, 28–31 October, by Craig Jones and three co-authors, all from the  University of Colorado.  The presentation (Paper No. 18-13) was titled “Exploring an Alternative Explanation for the Laramide Orogeny.”  The four authors of this presentation suggest that the Farallon plate hypothesis is an appealing visualization of the cause of the Laramide orogeny, but it has all kinds of problems: many facts do not fit the associated assumptions, and it is very hard to believe that this subduction pulling land and folding it together and thrusting pieces of it over existing terrain could go on for more than 20 million years.

The authors then propose their alternative.  Their alternative is a variant on the Farallon plate subduction idea, having it go only a little ways eastward, in essence, and shallower over that distance, under all of western North America.  This would make it a bit more like a collision rather than just a subduction beneath that grabs some roots along the way.  It would compress the land as observed, causing mountain building, but there is another wrinkle in this version: very deep rock under places like Wyoming interfere with the counterflow in the more malleable asthenosphere, causing land to fall down (subside) in front of this mass of compressed and higher, hence heavier mountain being created (the illustration to have in mind is the Colorado Rockies).  This, the authors suggest, may explain the basement structure of the plains at the front range of the Rocky Mountains.  I am not doing justice to the finer points of their arguments, they use big words, and it is a complicated picture with many forces and process they weave into their variant hypothesis.  Western North American geology is complicated, so there is nothing intrinsically wrong with that.  Their alternative also is used to explain the volcanic activity seen all over the west after the Laramide episode was over. They acknowledge that much more needs to be known before a definitive model for the Laramide orogeny can be agreed on.

SOME MORE BACKGROUND ON THE SEVIER AND LARAMIDE OROGENIES (for the curious)

Wikipedia, the free online encyclopedia, has this to say about the Sevier and Laramide orogenies at http://en.wikipedia.org/wiki/Sevier_orogeny :

Another web site, one about Colorado Geology, has a wonderful discussion of the Laramide orogeny here http://www.cliffshade.com/colorado/geo_overview.htm#shallow , and this is my digest and interpretation  of what it says about the cause of this orogeny, the orogeny that built the Rockies! Please do read the actual account on the linked site and see their great graphics with very good explanatory power.

The primary reconstruction of what happened to cause the mountain-building event known as the Laramide orogeny involves a 'plate,’ a slab of ocean bottom that is moving away from the spreading center from which it arose and is becoming heavier as it changes mineralogically and is pushed away from the spreading center.  As it moves away it starts to sink down and typically under-rides the continent or other land mass it may encounter.  The particular plate associated with the Laramide mountain-building event is the Farallon plate, and it has completely sunk into earth.  But it caused some mischief as it rode under much of western North America and became, in places, cemented, as it were, to overlying terrain.  As you can imagine, it would cause a chunk of land to be compressed as it pulled it forward, and when you compress you raise height.  Hence: mountains and lots of faults where the earth broke to allow some pieces of land to rise and others to fall.

Pages on this web site from 2005, 2006, and early in 2008 that had the wrong explanation for the Keystone thrust fault, not recognizing the role of the Sevier and Laramide orogenies, have been corrected and now refer forward to this March 2008 discussion.

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